An optical waveguide device comprising of an optical waveguide, a buffer layer and traveling-wave type electrodes which are integrated on a substrate having an electro-optic effect made of lithium niobate (LN) or the like, has been widely employed for long-haul and high-capacity optical fiber communication systems and optical measuring instruments. Particularly, an optical intensity modulator, an optical phase modulator and a polarization scrambler are frequently employed as a key device for optical fiber communication systems.
Recently, with the increase of data traffic due to the rapid advance of the Internet, the optical fiber communication system is required to have faster transmission speed and higher capacity. Then, in order to realize such a high-speed and high-capacity optical fiber communication system, it is required to repress the broadening of an optical pulse due to the slight optical dispersion in an optical fiber employed as a transmission line.
As a repressing method, it is proposed that the chirp degree in the optical fiber communication system is controlled by an optical phase modulator placed after an optical intensity modulator as a converter of an electric signal into an optical pulse and thus, improve the transmission performance of the system.
As mentioned above, there is a trend to install plural optical modulators in the optical fiber communication system, but with the increase of the number of optical modulators, the insertion loss is also increased. In this point of view, the demand for monolithic integration of plural optical modulators becomes higher because the insertion loss in the optical fiber communication system will be reduced by decreasing the substantial number of optical modulators, instead of a conventional optical communication structure in which plural optical modulators are joined by means of fiber splicing.
Such a monolithically integrated optical device is fabricated as follows. First of all, optical waveguides are formed on a given substrate according to the required function, such as optical intensity modulation and optical phase modulation. Then, a buffer layer is formed uniformly on the substrate, and electrodes are formed on the substrate via the buffer layer. As a result, an optical waveguide device on which an optical intensity modulator and an optical phase modulator are monolithically integrated is completed.
Namely, the optical intensity modulator and the optical phase modulator are fabricated on the same condition except their optical waveguide pattern.
In this case, however, the inherent performance of each modulator can not be often exhibited sufficiently, and therefore, the optical waveguide device can not achieve the designed characteristics. For example, when a DC voltage is applied to the optical intensity modulator so as to control the operation point, a relatively large DC drift may occur depending on the fabricating condition of the optical intensity modulator, so that the long-term reliability of the optical intensity modulator may be deteriorated. As a result, when the plural modulators are fabricated on the same substrate to complete an integrated optical waveguide device, as mentioned above, the long-term reliability of the resultant optical waveguide device may be deteriorated.